Measurements of fractionated gaseous mercury concentrations over northwestern and central Europe, 1995-99

Abstract

Although it makes up only a few per cent. of total gaseous mercury (TGM) in the atmosphere, the fraction of oxidised (divalent) mercury plays a major role in the biogeochemical cycle of mercury due to its high affinity for water and surfaces. Quantitative knowledge of this fraction present in mixing ratios in the parts-per-10(15) (ppq) range is currently very scarce. This work is based on approximately 220 data for divalent gaseous mercury (DGM) collected during 1995-99 in ambient air. Over the course of the measurements, the sampling and analytical methods were modified and improved. This is described here in detail and includes transition from wet leaching and reduction procedures to thermo-reductive desorption, the use of annular as well as tubular denuders and adoption of an automated sampling system. The concentration of DGM exhibited a strong seasonal behaviour in contrast to atomic gaseous mercury, with low values in winter and maximum values in summer. The DGM/TGM ratios were frequently found to be below the detection limit (< or = 1%) and in the range 1-5%. A trend of diurnal DGM patterns was observed and implies photolytically induced sources. Scavenging of DGM during rain events was also noticed.

Full text

Measurements of fractionated gaseous mercury concentrations over
northwestern and central Europe, 1995–99†
Jonas Sommar,* Xinbin Feng, Katarina Ga
˚rdfeldt and Oliver Lindqvist
Inorganic Chemistry, Department of Chemistry, Go
¨teborg University, 412 96 Go
¨teborg, Sweden.
E-mail: sommar@inoc.chalmers.se
Received 1st April 1999, Accepted 12th July 1999
Although it makes up only a few per cent. of total gaseous mercury (TGM ) in the atmosphere, the fraction of
oxidised (divalent) mercury plays a major role in the biogeochemical cycle of mercury due to its high affinity for
water and surfaces. Quantitative knowledge of this fraction present in mixing ratios in the parts-per-1015 (ppq)
range is currently very scarce. This work is based on #220 data for divalent gaseous mercury ( DGM ) collected
during 1995–99 in ambient air. Over the course of the measurements, the sampling and analytical methods were
modified and improved. This is described here in detail and includes transition from wet leaching and reduction
procedures to thermo-reductive desorption, the use of annular as well as tubular denuders and adoption of an
automated sampling system. The concentration of DGM exhibited a strong seasonal behaviour in contrast to
atomic gaseous mercury, with low values in winter and maximum values in summer. The DGM/TGM ratios were
frequently found to be below the detection limit (∏1%) and in the range 1–5%. A trend of diurnal DGM patterns
was observed and implies photolytically induced sources. Scavenging of DGM during rain events was also noticed.
Dry adsorption methods to trap oxidised mercury in flue
Aim of investigation
gases rely on the same principle. Oxidised mercury, ‘HgCl2’,
Mercury is the only air pollutant predominantly present in is complexed to [HgCl3]−or [ HgCl4]2−and retained as the
atomic form (Hg0). Owing to high detection sensitivity, it is complex anion in a KCl matrix. The diffusion denuder
possible to monitor roughly background sub-ppt concen- method10 separates gases from particles based on the fact that
trations in the atmosphere with direct absorption methods.1they diffuse much faster than particles. For sampling purposes,
For oxidised mercury forms present in the parts-per-1015 (ppq) the gas has to be sucked along a surface that acts as a sink
range in ambient air, accumulative sampling is required. for the specific gas of interest. Numerous coatings can act as
Besides CH3HgX, (CH3)2Hg and Hg0, speciation of gaseous a sink for gaseous divalent mercury compounds but not for
mercury is not obtained, only fractionation. Recently, efforts elemental mercury, such as alkali metal hydroxides, halides
have been made to determine the fraction of oxidised (divalent) and chromates.11 The use of KCl is mainly due to its high
gaseous mercury in the atmosphere.2–5 Normally, this is rep- deliquescence point. The compound traps DGM compounds
resented by reactive gaseous mercury (RGM ). The term is, such as HgCl2and CH3HgCl.2
however, media-dependent and accordingly depends on HgIIaWe have previously published a paper on the applicability
in the aqueous phase, defined by Brosset.6In the following, of KCl denuders to sample and determine RGM in ambient
divalent gaseous mercury ( DGM ) will be used instead and air.2In the present paper, a data set of ambient air RGM,
only when it is appropriate will the term RGM be given. Even DGM and TGM samples from 1995 to date is presented. Over
though DGM makes up only a small portion of the total the course of the measurements, the method was modified and
gaseous mercury ( TGM ) in the atmosphere, it plays an improved in order to make the sampling less strenuous and
important role in the biogeochemical cycle of mercury in the time-consuming and achieve higher time resolution. This
environment due to its high solubility in water and easy includes transition from wet leaching and reduction procedures
deposition back to terrestrial ecosystems by both wet and dry to thermo-reductive desorption,12 the use of annular as well
processes. DGM exhibits dry deposition velocities similar to as tubular denuders and modifications of the technique aimed
those of HNO3
4while the corresponding figure for Hg0is at integration and automation. The different techniques are
several orders of magnitude lower. Modelling studies over briefly described and commented upon, being used in overlap
various spatial scales have shown that even trace amounts of during transition periods.
DGM species may control the overall deposition of mercury.7,8
However, further advancement of mercury modelling is limited
by the lack of data on the mercury composition in the
Description of experimental procedures
atmosphere. KCl coated denuders
The earliest evidence for the presence of ppq concentrations
of RGM in ambient air was presented by Brosset.9Using a The denuders were made from quartz or borosilicate glass
high flow refluxing mist chamber, Stratton and Lindberg3,4 tubes. In addition to tubular denuders (Ø#6 mm), annular
were able to obtain diurnal-resolved RGM in ambient air. denuders consisting of two coaxial quartz tubes with #1mm
Representative samples collected corresponded to a detection spacing and with sandblasted annulus walls were also
limit of 1 ppq RGM in ambient air. employed. The cleaning of the tubes and application of a
methanolic KCl solution to the walls were performed as
described previously.2Both ends were cleaned with doubly
†Presented at AIRMON ’99, Geilo, Norway, February 10–14, 1999.
J. Environ. Monit., 1999, 1, 435–439 435
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de-ionised water (Millipore, Milli-Q). The effective KCl coated always present. The generator effluent was then diluted to a
suitable concentration by adding carrier gas.length was 50 and 20 cm for tubular and annular denuders,
respectively. The denuders were repeatedly ‘blanked’ by heat-
ing in mercury-free inert gas until low long-term blanks were Sampling set-up
obtained. Denuders that were to be exposed were generally The collection efficiency of the divalent species has been shown
‘blanked’ shortly before collection started. to be near-quantitative.2Therefore, most of the DGM and
RGM measurements were performed with single collectors
Determination of mercury only. Usually, about 1–1.5 m3samples were collected in the
Divalent gaseous mercury. Mercury trapped in the denuders KCl denuder. The tubular and annular denuders were main-
was determined either by leaching or thermal desorption. tained at a flow of 0.7–1 and 6 –9 l min−1, respectively. The
Borosilicate denuders were only analysed by the former method denuders were mounted vertically with a downturned funnel
whereas denuders made from quartz were subject to testing at the inlet to protect them from precipitation. In about 60%
with both methods. The extraction solution, suprapure grade of the sampling events TGM was co-sampled with DGM.
HCl (Merck), was diluted to 1.0 M with Milli-Q water and
further purified following the procedure previously described2Manual sampling. The assembly for manual sampling of
before being used. Divalent mercury molecules were extracted divalent mercury is given in Fig. 1. Denuders mounted verti-
as their chloro complexes. The leachates were analysed for cally at least 1 m above ground were generally heated at
both RGM and DGM. DGM was obtained after oxidising 40 –50 °C to avoid KCl deliquescencing during sampling. By
the leachate with BrCl followed by reduction with having a negligible pressure drop, the volume exposed to
NH2OH·HCl and eventually with SnCl2and determined by the denuder was obtained by simply using a gas meter
cold vapour atomic fluorescence spectrometry (CVAFS) (Schlumberger) connected downstream.
(Brooks Rand CVAFS-2 Mercury Analyzer or PSA
Millennium Merlin Mercury Analyser). In the former case, a Automated sampling. An automated sampling system for
dual amalgamation step was employed. DGM based on the thermo-reduction desorption principle was
The set-up used to determine mercury by thermal desorption also used. The configuration shown in Fig. 2 is generally
is described below. During analysis or ‘blanking’ of the KCl adopted from the Tekran Model 1130P system (Tekran,
denuder, it was heated externally from a NiCr resistance Toronto, Canada). A heated ( 50 °C) annular denuder with the
ribbon. The temperature (<700 °C) and time were set to the effective KCl coated length enclosed in an oven equipped with
grade of contamination of the denuder. By desorbing at 450 °C, a cooling fan was used to collect DGM. The denuder was
generally >90% of mercury trapped was released in a few connected with short, cleaned Teflon transfer lines to a pump
minutes.12 Samples were generally treated at 450 °C for 10 min. and CVAFS (Tekran 2537A Mercury Vapour Analyzer) unit.
Collectors made of borosilicate were also heated to a few The working principle of the CVAFS system has been described
hundred degrees to attain low blanks. During sample analysis, by Schro
¨der et al.14 The surface temperature of the denuder
mercury species desorbed were pyrolysed before preconcen- was controlled with a set-point controller ( Eurotherm 2416 )
tration on an analytical column with gold-coated quartz beads including time-regulated heating and cooling plug-ins. The
secured with quartz wool plugs. The pyrolyser consisted of flows through the denuder were synchronised with the tempera-
quartz beads in a quartz column maintained at 900 °C. The ture as well as with the CVAFS unit’s sampling cycles using
gold trap was heated in a short pulse to 500 °C and atomic Tekran accessories (1110 Synchronized two port sampling
mercury released was detected by CVAFS (Brooks Rand unit, 1120 Standard Addition Controller). This unit was
CVAFS-2 Mercury Analyzer or Tekran 2537A Mercury continuously sampling ambient air at 1.5 l min−1. During
Vapour Analyzer). The output signal from the detector of the preconcentration of DGM, when an additional gas flow of
PSA or Tekran instrument was acquired on a portable #4.5 l min−1passes through the denuder, the CVAFS unit
computer. simply detects Hg0. A few cycles before desorption, the inlet
of the denuder was blocked with mercury-scrubbed ambient
Total gaseous mercury. TGM was collected on gold filled air. Hence, the detector only experienced the influence of
quartz tubes (gold traps) either manually or automatically by
a field-portable instrument ( Ekoservis Gardis-1A or Tekran
2537A Mercury Vapour Analyzer). The gold traps were ana-
lysed as described above. Calibration of the instruments was
achieved by injecting certain amounts of elemental mercury
into the analytical system. Mercury was delivered as acidic
aliquots into the PSA instrument and as nitrogen gas saturated
with mercury into the other instruments. The Tekran instru-
ment also exhibits an internal calibration system with a
permeation source.
Laboratory testing of KCl denuders
A detailed description of the HgCl2generation system has
been given previously.2The generation rate is highly dependent
on the temperature and flow of the carrier gas. The parts of
the system upstream of the delivery point have to be completely
heated to a temperature exceeding that of the bath to avoid
condensation. The composition and stability of the source
were tested by connection to an on-line fractionation system
normally used for flue gases.13 The gas stream was pyrolysed
at 600 °C in a quartz cell and detected by Zeeman-effect
electrothermal AAS (Semtech 2000 Mercury Analyzer) at the
Fig. 1 Schematic diagram of set-up for manual sampling with
denuders.
same temperature. A few per cent. of elemental mercury was
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Fig. 2 Schematic diagram of set-up for automated fractionation of
gaseous mercury. The flow indicated by the dark-coloured lines to
the left of the denuder is active during preconcentration while the
flow indicated by the light-coloured lines acts to block the denuder
from ambient air during thermal desorption.
mercury zero air. During desorption, the temperature of the
denuder was ramped to 450 °C and, consequently, DGM
trapped was released, pyrolysed and detected. After the
denuder had been allowed to cool down, preconcentration was
resumed. The internal blank of the denuder was intermittently
checked by passage of mercury zero air during the whole
Fig. 3 Locations of European sampling sites.
sampling period followed by thermal desorption. In the most
recent measurements, deposition of coarse particles on the
denuder was prevented by passing ambient air through a Analytical performance of the methods
cyclone (2.5 mm aerodynamic diameter cut-off).
During 1996, two parallel sampling lines were run with dual
leaching analysis and the variations were in the range up to
Sampling sites 30%. The DGM method blank is generally 15–35 pg and as
Air samples were taken manually from 1995 and automatically low as 10 pg can be obtained for RGM if care is taken to
since 1998. Sampling was performed during two international avoid every possible contamination. Leaching of denuders
intercomparison exercises at Mace Head, County Galway, treated with or without repeated heating after a new coating
Ireland, and at Sasseta, Tuscany, Italy, during September 1995 had been applied showed that heating was necessary to elimin-
and June-July 1998, respectively, involving groups from North ate initial contamination. The use of borosilicate tubes, how-
America and Europe. ever, prevented high temperatures from being used. As it was
The sampling sites in Sweden were located around the city a component of all the solutions used, distillation of the
of Go
¨teborg (58°N12°E) situated on the west coast. About suprapure grade HCl from mercury induced by SnCl2addition
65 samples were exposed at the Brottka
¨rr site 15 km south- was also found to be important.2Contribution from reagents
west during 1995–97. About 25 samples were collected outside gives the DGM method a 5–20 pg higher blank compared
the building of the Department of Chemistry in the urban with that of RGM. The detection limit, based on 3sof the
area of Chalmers. The S:t Jo
¨rgen site is located 15 km north, method blanks, varies between 5 and 15 pg, corresponding to
where a combustion simulator is operated with mercury injec- air concentrations of <2 ppq.
tion for research purposes. Samples were taken outside the The analytical precision of field samples by the thermal
simulator building. Measurements of DGM were also per- method was similar to that described above ( 5–40% variability,
formed within the Mercury Over Europe/Mediterranean triplicate samples, n=7 ). The regenerative use of thermal
Atmospheric Mercury Cycling (MOE/MAMCS) Project. denuders makes the blank smaller and less variable without
MOE/MAMCS is a multi-year project set-up in 1998, which influencing the collection efficiency. Typical values of the blank
includes air sampling at ten sites in Western and Southern and detection limit were a few picograms for each based on
Europe. Some results are reported from the Ro
¨rvik site located field blanks. The internal blank of the annular denuder
35 km south-west of Go
¨teborg. was higher, but could be suppressed to 5–10 pg during auto-
The locations of the European sampling sites are shown mated sampling with repeated heating cycles. The detection
in Fig. 3. limit of the thermal methods corresponds typically to DGM
concentrations of <1 ppq.
The sampling data were screened from outliers. Those
Results and discussion
generally encountered resulted from passivation of gold traps
during field measurements and mainly influenced TGM (alsoIn the following, the concentrations given were calculated
based on the molar weight of atomic mercury; hence, the DGM during automated sampling), giving reduced concen-
trations of <100 ppq of the former. About 10% of themixing ratios are slightly overestimated for RGM and DGM.
TGM data presented here were averaged to cover RGM/DGM automated and <2% of the manually collected RGM/DGM
samples were rejected.sampling periods.
J. Environ. Monit., 1999, 1, 435–439 437
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Table 1 Summary of mixing ratios of RGM, DGM and TGM obtained in ambient air at European sites, 1995–1999. The values in italic type
indicate that most of the single samples grouped together were below the detection limit given in Fig. 4
RGM( ppq ) DGM( ppq ) DGMa/TGM (%)
Site Season Method x
:±snx
:±snx
:±sn
Brottka
¨rr All Manual leaching 3.4±2.9 45 4.2±4.0 19 1.7±1.6 32
Summer 4.3±3.1 30 5.0±4.2 15 2.2±1.7 19
Winter 1.5±1.0 15 1.3±0.6 40.8±0.7 13
Ro
¨rvik Winter Manual heating 2.6±2.4 30 1.1±0.9 20
Winter Automated heating 1.0±1.0 34 0.5±0.7 34
Summer Manual heating 3.0±3.1 14 1.7±1.8 13
Summer Automated heating 3.9±2.1 21 2.5±1.3 21
Chalmers Summer Manual leaching 16.0±2.1 3 6.9±2.1 3
Summer Manual heating 5.3±2.5 10
Winter Automated heating 1.0±0.4 13 0.4±0.2 13
S:t Jo
¨rgen Summer Manual leaching 8.7±5.4b20b
Mace Head Summer Manual leaching 9.1±2.9 4 3.0±0.9 4
Sasseta Summer Manual heating 2.1±0.9b14b
aIn Brottka
¨rr data, RGM/TGM is displayed. bIncludes duplicate or triplicate exposed samples.
Intercomparison with other methods to determine oxidised the samples were analysed for both RGM and DGM. The
latter was found to be higher using a one-way analysis ofgaseous mercury
variance test (F=0.46, P<0.06). RGM and DGM were well
The methods currently available to fractionate/speciate correlated with each other (r=0.92, P<0.0001 ) but not with
mercury in ambient air—denuder-based techniques and mist TGM (P=0.21). As shown in Table 1, the difference was more
chambers—were intercompared during the start-up phase of pronounced during the summer period. The RGM fraction is
the MOE/MAMCS project in Sasseta. Owing to the scarce considered to represent inorganic compounds, since even the
data set, it was difficult to rank the different methods. However, most stable inorganic mercury compounds in aqueous solution
it can be concluded that they generally produce relatively have been found to be easily reduced by SnCl2. On the other
similar results with variability up to 30–40%. The complete hand, CH3HgCl(aq) is not reduced by SnCl2and does not
data set including TGM and particulate Hg data will be belong to the RGM fraction.15 This implies the presence of
published elsewhere.15 The results obtained by manual tubular gaseous monomethylated mercury species in ambient air.
and automated annular denuders were intercompared during However, other methods which can exclusively identify such
one campaign at Ro
¨rvik. The data were significantly correlated compounds in air (mist chambers15 and graphitised carbon
(P<0.10, n=9) but differed occasionally in magnitude (within traps16,17 ) have to be used during such speciation.
a factor of 2). The results obtained with annular denuders The concentration of oxidised gaseous mercury exhibited a
were consistently lower. strong seasonal behaviour (P<0.01) with low values in winter
and maximum values in summer. The concentration during
Concentrations of RGM and DGM in air the winter period often dropped below the detection limit (1–2
ppq). This trend has also been observed by Lindberg
In Table 1, the complete set of over 200 data is categorised
and Stratton4and was attributed to seasonal differences in
into location, season and analytical method. The most extens-
air stagnation and chemical kinetics. Extensive continental
ive data set is from Brottka
¨rr and covers three years, and is
European TGM measurements imply that Hg0is predomi-
displayed in Fig. 4. Exposed denuders were analysed after
nantly sink- rather than source-modulated,6,9,18 –20 which is
manual leaching. RGM was determined throughout the whole
characteristic of atmospheric trace gases removed by oxidation
campaign while DGM was determined occasionally. Samples
processes. Higher oxidant concentrations during summer lead
were usually exposed on a diurnal basis with a flow rate of
to faster oxidation and to a summer minimum of TGM. The
about 700 ml min−1. The average level of DGM measured
intensified oxidation process contributes to increased concen-
was 4.2 ppq compared with 3.4 ppq for RGM. Nineteen of
trations of DGM and subsequently enhanced deposition21, 22
of the highly surface-reactive oxidised mercury forms.
However, the role of different photolytically induced gas-phase
oxidants has not been shown in any detail.23 In its extreme
manifestation, when high concentrations of oxidants build up
during polar sunrise in the Arctic, fast depletion of elemental
gaseous mercury occurs.24
During winter periods, the frequency of precipitation
increases from 20 to 40% and leads to enhanced scavenging
of oxidised mercury. RGM/DGM was measured during three
rainfall events during summer 1996 and indeed oxidised gase-
ous mercury was found to decrease on average by #60% for
both fractions. TGM showed no consistent variation during
the same period of time. The limited amount of oxidised
mercury collected on the tubular denuders did not allow any
diurnal resolution of the measurements. The introduction of
high-flow annular denuders enabled the same amount to be
collected during a few hours. Fig. 5 shows automated 6 h
measurements of DGM and Hg0at the Chalmers site during
Fig. 4 Time–concentration behaviour of RGM/DGM at Brottka
¨rr
the shift between January and February 1999 (a data series
during 1995–97. The bars given indicate detection limit for each
fraction.
without any significant passivation of gold traps).
438 J. Environ. Monit., 1999, 1, 435–439
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definitely not the case for RGM/DGM. As it is being measured
extensively within the MOE/MAMCS campaign and else-
where, quantitative knowledge of RGM/DGM is, however,
likely to be extended in the near future.
Acknowledgements
This study was initiated by Zifan Xiao, whom we greatly
acknowledge together with Shiqiang Wei. This research was a
contribution to the ‘Mercury species over Europe’ project
being carried out in the specific RTD program sponsored
by the European Community under contract number
ENV4-ET97–0595.
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J. Environ. Monit., 1999, 1, 435–439 439
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